Shrinkage, cracking, voids, pores, poor density, and lamination or skin formation are problems in the art of molding. Shrinkage is a decrease in volume of the material being molded as the material cools from a molten to a solid. Cracking is the result of shrinkage; cracking is the formation of cracks in the cooled and solid molded product. Porosity may also be caused by shrinkage. Voids, pores, and poor density may further be caused by turbulent flow of the molten material. Simply pouring molten material is relatively highly turbulent. Lamination is the formation in a molded product of two or more layers of the same material; the two or more layers are caused by different portions of the molten material cooling at different times. One type of lamination is the formation of skin. This occurs when the molten material which is in contact with the relatively cool cavity wall cools prior to the molten material not in contact with cavity surfaces. One area where the problem becomes acute is in the formation of lead battery terminals for batteries as any voids or cracks in the battery terminals can cause the terminals to leak acid.
Pouring causes problems other than poor density. For example, pouring--especially hand pouring--is slow. Too much time elapses before solidification progresses to the point of no movement for pressure to be brought to bear timely on the metal in the cavity.
Accordingly, general objects of the invention are to provide a unique method and apparatus for molding metal which minimizes the number of pores and voids in the final product and maximizes the density of and a smooth finish to the final product.
Another object of the invention is to provide such a method which uniquely includes the step of feeding without pouring a charge of molten metal to a location between a punch and die cavity. A shuttle slides a shot or charge of the molten material from a reservoir to the punch and die cavity. Such contributes to the formation of a product which is air free and dense.
Another object of the invention is to provide such a method which uniquely includes the step of elevating a charge of molten material into a mold cavity to avoid a pouring of the molten material and minimize the turbulence of the charge. A punch is driven vertically and relatively quickly and gently to lift the charge into the mold cavity without any turbulence or atomization and with minimal loss of heat. Such further contributes to a product which is air free and dense.
Another object of the invention is to provide such a method which uniquely includes the step of immediately applying pressure to the molten material. Pressure is applied to the molten material within one-quarter of a second after the material has been elevated into the cavity and prior to the solidification of any of the molten material.
Another object of the invention is to provide such a method which uniquely delivers a precise quantity to a mold cavity. The shot or charge of molten material is measured twice, once when the charge is collected from a reservoir of molten material and a second time when a punch elevates the charge into the charge cavity. Accordingly, pressure can be applied to the precisely measured charge immediately after the charge has been introduced to the cavity, machining of the final product is minimized, and molten metal is conserved.
Another object of the invention is to provide such a method which is uniquely carried out in an air-free environment. Specifically, charges of molten material are collected from below the surface of a reservoir of the molten material, then transferred to the punch and die assembly with minimal exposure to air, and then elevated into the die cavity under vacuum.
Another object of the invention is to provide a unique reservoir and shuttle assembly for loading a charge of molten material on the shuttle for transfer to the punch and mold assembly. The shuttle includes a through hole which communicates with the inside of the reservoir below the surface of the molten material. Molten material flows upwardly into the hole from the bottom opening of the hole to minimize a pouring of the material. The shuttle then slides the charge out of the reservoir to the punch and mold assembly. While sliding back and forth between the reservoir and punch and mold assembly, the shuttle remains sealingly engaged with a wall of the reservoir.
Another object of the invention is to provide such a method which uniquely retains the heat of the charge until the step of applying pressure to the molten material. Specifically, the disc shape of the body of the charge minimizes the amount of surface area exposed to further minimize the loss of heat during the steps of feeding and elevating. The punch and shuttle are driven sufficiently quickly from a reservoir of molten material to minimize the loss of heat from the charge.
Another object of the invention is to provide metal products having thin walls of less than one-eighth of inch in thickness and having a smooth finish and high density.
These and further objects and advantages of the present invention will become clearer in light of the following detailed description of an illustrative embodiment of this invention described in connection with the drawings.